Cherry Pick Add public API to trigger Login for Admin flow. (#4026)

[brandonpage]

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[github-actions]

	1 Error
🚫	Please re-submit this PR to the dev branch, we may have already fixed your issue.

Generated by 🚫 Danger

[github-actions]

	1 Warning
⚠️	Static Analysis found an issue with one or more files you modified. Please fix the issue(s).

Clang Static Analysis Issues

File	Type	Category	Description	Line	Col
SFUserAccountManager	Nullability	Memory error	Null passed to a callee that requires a non-null 2nd parameter	1579	15
SFUserAccountManager	Nullability	Memory error	Null passed to a callee that requires a non-null 2nd parameter	1594	15
SFUserAccountManager	Nullability	Memory error	nil passed to a callee that requires a non-null 2nd parameter	2229	13

Generated by 🚫 Danger

[github-actions]

	Tests	Passed	Skipped	Failed ❌️
AuthFlowTester UI Test Results all	1 ran			1 ❌

Test	Result
AuthFlowTester UI Test Results all
AuthFlowTesterUITests.xctest
LegacyLoginTests.testCAOpaque_DefaultScopes_WebServerFlow()	❌ failure

• View AuthFlowTester UI Test Results all

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.37%. Comparing base (fe97c22) to head (5d3b879).
⚠️ Report is 343 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4028      +/-   ##
==========================================
+ Coverage   63.35%   65.37%   +2.01%     
==========================================
  Files         250      260      +10     
  Lines       21659    22533     +874     
==========================================
+ Hits        13723    14730    +1007     
+ Misses       7936     7803     -133     

Components	Coverage Δ
Analytics	70.78% <ø> (ø)
Common	69.75% <66.66%> (+0.35%)	⬆️
Core	57.31% <71.13%> (+3.52%)	⬆️
SmartStore	74.70% <100.00%> (+0.70%)	⬆️
MobileSync	87.41% <ø> (ø)
see 37 files with indirect coverage changes

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[github-actions]

	Tests	Passed ✅	Skipped	Failed
SalesforceSDKCore iOS ^18 Test Results	627 ran	627 ✅

Test	Result
No test annotations available

• View SalesforceSDKCore iOS ^18 Test Results

[github-actions]

	Tests	Passed ☑️	Skipped	Failed ❌️
SalesforceSDKCore iOS ^26 Test Results	627 ran	625 ✅		2 ❌

Test	Result
SalesforceSDKCore iOS ^26 Test Results
testNonCallbackURL()	❌ failure
SFUserAccountManagerTests.testAuthHandler	❌ failure

• View SalesforceSDKCore iOS ^26 Test Results